1. Field of the Invention
The present invention relates to an amplifying device.
2. Description of the Related Art
In control systems for various fields, analog signals detected by analog sensors have a minute amplitude or low level of current/voltage. Accordingly, in order to amplify the signal to a level that the control system can handle internally, a first-stage amplifier (preamplifier) is provided.
As an example of the control system having such an amplifier, FIG. 12 illustrates an electret condenser microphone (ECM) system that is mainly used as a microphone of a mobile phone on the talking partner side. See, for example, Japanese Patent Application Laid-Open Publication No. 2003-243944.
The ECM system is configured to have a three-terminal amplifier 900 that is an amplifier having a Vin terminal 901, a Vdd terminal 902, and a Gnd terminal 903 wherein a fixed electrode of an electret condenser 203 having its conductive thin film grounded is connected to the Vin terminal 901, a power supply line 201 is connected via a load resistor 200 of resistance R1 to the Vdd terminal 902, and the Gnd terminal 903 is grounded.
The electret condenser 203 has the conductive thin film and the fixed electrode opposite each other. The conductive thin film is charged beforehand by a direct-current power supply of about several tens to 100 volts, and when a voice signal as a condensation-rarefaction wave of air is transmitted to the conductive thin film, the thin film vibrates. Thus, the capacitance Cmic (about 3 pF) of the electret condenser 203 varies thereby producing an alternate voltage waveform (hereinafter, called an alternating signal) according to the variation of the capacitance Cmic. The alternating signal usually has its DC component being at ground potential and a minute amplitude of about several tens mV.
The three-terminal amplifier 900 comprises a junction electric field effect transistor (hereinafter, called a J-FET) 904 having its gate electrode connected to the Vin terminal 901, its drain electrode connected to the Vdd terminal 902, and its source electrode connected to the Gnd terminal 903 thus having the source grounded, and a resistor 905 connected between signal lines respectively leading to the gate electrode and to the source electrode to stabilize the potential of the gate electrode to ground potential.
Here, the following input characteristics are required of the three-terminal amplifier 900 in terms of the input resistance (the resistor 905 and the like) and input capacitance (a parasitic capacitance of the J-FET 904 and the like).
First, the input resistance is required to be of a high resistance value. The electret condenser 203 and the input resistance form a high band pass filter, and in order that a voice signal of about 100 Hz in an audible band is input to the three-terminal amplifier 900 without being attenuated, the resistance of the resistor 905 needs to be of about several hundreds M to several GΩ. Note that letting Rin be the value of the input resistance, the cutoff frequency f1 of the high band pass filter is defined by the following equation 1,f1=1/(2×π×Rin×Cmic).  (1)
For example, if the capacitance Cmic of the electret condenser 203 is 3 pF and the cutoff frequency f1 is 50 Hz, the value of the input resistance Rin becomes 1061 MΩ.
Next, the input capacitance is required to be of a low capacitance value. That is, the input capacitance of the three-terminal amplifier 900 is connected in series to the electret condenser 203. Thus, if the value of the input capacitance is large, an alternating signal occurring across the electret condenser 203 will be divided such that the level of the alternating signal at the input of the three-terminal amplifier 900 is reduced. Furthermore, because the value of the input resistance is large, also from the point of view of suppressing the time constant defined as the product of the input resistance and the input capacitance, the input capacitance is required to be of a low capacitance value.
Adopted as an amplifier element used in the three-terminal amplifier 900 is a source-grounded J-FET 904 excellent in balance of high input resistance, low input capacitance, and low noise. Note that letting gm be the conductance of the J-FET 904 and ATTin be an attenuation amount due to the input capacitance and the like, the amplification gain Av of the source-grounded J-FET 904 is defined by the following equation 2:Av=gm×Rl−ATTin.  (2)
The amplification gain of the J-FET varies with the conductance gm of the J-FET as shown by equation 2. However, the conductance gm of the J-FET usually varies possibly by about −50 to 200%, and thus conductance gm's are classified into ranks for a management purpose. Even for J-FETs of the same type, variation in the conductance gm causes variation in the amplification gain of the J-FETs, and thus, variation in the amplification gain of amplifiers using the J-FETs (such as three-terminal amplifiers 900) occurs.
Furthermore, as shown in FIG. 13, when the amplification gain of a J-FET is increased, a drain saturation current Idss (about 200 μA) is also to be increased. As the drain saturation current Idss increases, consumption current also increases. Hence, the increase in the amplification gain of the J-FET is restricted for the reason of consumption current. That is, there exists a trade-off relationship between the increase in the amplification gain of a J-FET and reduction in consumption current.
As such, J-FETs as amplifier elements used in amplifiers are excellent in balance of high input resistance, low input capacitance, and low noise, but have the problem that their amplification gain is difficult to adjust.